Keggin-type polyoxometalates (phospho and silicomolybdic
based HPAs) in an amount of 5 mol % have shown to be excellent
acid catalysts for the one-pot synthesis of 3,4-dihydropyrimidinones
in good yields, using an excess of aldehyde. The theoretical
calculations have confirmed the mechanism of the reaction
including the formation of the so called ‘ureido-crotonate’.
Tungsten-based Dawson POMs a-K6P2W18O62, b-K6P2W18O62 and
K6P2W12Mo6O62, in an amount of 5–8 mol % are less acidic and give
lower yields than the Keggin-type POMs.